1. Field of the Invention
This invention relates to a real image mode finder optical system suitable for use in a lens shutter camera or an electronic still camera which is constructed so that a photographing optical system is independent of a finder optical system, and in particular, to a real image mode finder optical system which has a large angle of emergence and is best adapted for mounting to a compact camera.
2. Description of Related Art
In general, finders constructed to be independent of photographing optical systems, used in lens shutter cameras, are roughly divided into two classes: virtual image mode finders and real image mode finders.
The virtual image mode finder has the advantage that an image erecting optical system is not required, but has the disadvantage that since an entrance pupil is located at the same position as an observer's pupil, the diameter of a front lens must be increased or the area of a visual field is not defined. An Albada finder of this type allows the area of the visual field to be definitely set, but has the problem that a half mirror coating is applied to the surface of a lens and thus the transmittance of the lens is reduced or flare is increased.
In contrast to this, the real image mode finder is such that the position of the entrance pupil can be located on the object side, and hence the diameter of the front lens can be decreased. Moreover, by placing a field frame in the proximity of the imaging position of an objective lens, the area of the visual field can be defined without reducing the transmittance.
A conventional real image mode finder, however, dose not provide a sufficient angle of apparent visual field (hereinafter referred to as an angle of emergence). Specifically, an object to be observed can be viewed only in small size. Thus, when the object is a person, there is the problem that it is difficult to view the expression of the person. A finder with a relatively large angle of emergence is disclosed, for example, in each of Japanese Patent Preliminary Publication Nos. Hei 6-51201 and Hei 11-242167. However, even such a finder does not provide a sufficiently large angle of emergence.
A so-called telescope has a large angle of emergence. However, the telescope, which has a high magnification, namely a small angle of visual field, cannot be applied to a finder constructed to be independent of the photographing optical system, used in a common lens shutter camera which has a wide angle of view.